Galanthamine, ((−)-[4aS-(4aα,6β,8aR)]-4a,5,9,10,11,12-hexahydro-3-methoxy-11-methyl-6H-benzofuro[3a,3,2,-ef][2]benzazepine-6-ol) (I),
is a tetracylic alkaloid which reversibly inhibits cholinesterase. Its effects are similar to those of physostigmine and neostigmine, but its inhibitory effect is lower. However, this drawback is counterbalanced by a broader therapeutical spectrum and lower toxicity. Galanthamine is used for the treatment of narrow-angle glaucoma, intoxications, nicotine and alcohol dependency and different pathologies of the nervous system, such as the Alzheimer's syndrome. For clinical purposes, it is administered in the form of hydrobromide, whose preparation was first described in the fifties (N. F. Proskumina et al., Zhur. Obshchei Khim. 22, 1899 (1952)).
Galanthamine can be prepared by total synthesis, but due to the presence of three chiral centres, the methodologies are particularly complicated. More commonly, galanthamine is isolated from plants of the Amaryllidaceae family, for example plants of the genus Galanthus, Crinum, Leucojum and Narcissus, which also contain several galanthamine-related compounds. In these plants galanthamine can be present in traces or in the maximum amount of 0.3%. However, many of these plants are protected species and therefore the recovery of galanthamine on an industrial scale requires the use of biomasses obtained from cultivations.
The presence of a relevant number of galanthamine structurally-related compounds makes it troublesome and often expensive to recover galanthamine in a purity suitable for the pharmaceutical use, in particular for the preparation of galanthamine hydrobromide.
The preparation of mixtures of galanthamine and related compounds from plant materials is generally carried out using conventional methods for the extraction of alkaloids, which comprise wetting the plant material with alkali solutions suitable for hydrolysing the alkaloid salts contained in the biomass to free bases and extracting with solvents wherein the alkaloid bases are soluble.
In the particular case of galanthamine-containing plants, the alkali solutions are solutions of inorganic bases such as sodium, calcium, potassium hydroxide or carbonate or solutions of ammonium hydroxide. Water-miscible solvents such as methanol, ethanol and acetone, or water-immiscible solvents, such as aliphatic and aromatic hydrocarbons or esters, for example ethyl acetate, can be used as extraction solvents. Extractions can be carried out at temperatures ranging from 20° C. to the boiling temperature of the solvent. Preferably, concentrated aqueous solutions of sodium carbonate can be used for the hydrolysis of the alkaloid bases; for the extractions, toluene at temperatures ranging from 20 to 70° C. can be used. With respect to the other solvents mentioned above, toluene has the advantage of satisfactorily exhausting the alkaloids while avoiding the extraction of polar constituents which would make the subsequent purification steps troublesome.
The alkaloids contained in the toluene extracts can be separated from the non-alkaloid components by extraction with an acidic aqueous solution, for example a 2% sulfuric acid solution, followed by alkalinization with sodium or potassium hydroxide, or sodium or potassium carbonate and single re-extraction with toluene, whereby the alkaloid components are obtained as a mixture.
These procedures allow to obtain from Amaryllidaceae complex mixtures of alkaloids having a galanthamine content ranging from 30 to 40%. There is still therefore the need for a purification process which provides galanthamine suitable for pharmaceutical use.